New U-Pb age constraints for the Laxford Shear Zone, NW Scotland: evidence for tectono-magmatic processes associated with the formation of a Paleoproterozoic supercontinent

The Lewisian Gneiss Complex in north-west Scotland is a part of the extensive network of Archaean cratonic areas around the margins of the North Atlantic. It is considered to be made up of a number of terranes with differing protolith ages, which have been affected by a range of different metamorphic events. A major shear zone, the Laxford Shear Zone, forms the boundary between two of these terranes. New dates presented here allow us to constrain the timing of terrane assembly, related to the formation of Palaeoproterozoic supercontinents. Early deformation along the Laxford Shear Zone, and primary accretion of the two terranes, occurred during the Inverian event at c. 2480 Ma. This was followed by extension and the intrusion of the mafic Scourie Dykes. Subsequently, renewed silicic magmatic activity occurred at c. 1880 Ma, producing major granite sheets, considered to have formed as part of a continental arc. A further collisional event began at c. 1790 Ma and was followed by slow exhumation and cooling. This Laxfordian event caused widespread crustal melting, metamorphism and deformation, and is considered to represent the final assembly of the Lewisian Gneiss Complex within the major supercontinent of Columbia (or Nuna).